Generally, polylactic acid as a plant-derived resin has a high specific gravity and practically has difficulty in obtaining a sufficient sound velocity of not lower than 1800 m/s, and hence the polylactic acid has not been used for a speaker diaphragm. As a material characteristic required for the speaker diaphragm, a sound velocity C (m/s) is often used. Using a Young's modulus E and a density ρ as important characteristics for the diaphragm, a calculating formula for determining the sound velocity C can be expressed by: C=(E/ρ)0.5. In other words, when a material is hard and lightweight (or has large rigidity and low density) as being ideal conditions for the diaphragm material, a value of the sound velocity C is large, and that is more ideal.
There is generally a correlation between the sound velocity C and a high limit frequency of the speaker, in which the speaker is required to reproduce sound up to about 20 kHz as an audible frequency threshold for humans, and for satisfying this requirement, the sound velocity C is required to be not lower than 1800 m/s.
With recent petrochemistry-free trends, a speaker diaphragm, formed by mixing kenaf fibers into polylactic acid as a plant-derived resin to improve sound velocity, has been studied. In the case of mixing the kenaf fibers into the polylactic acid as the plant-derived resin, the diaphragm becomes apt to break when a wt % of the kenaf fibers exceeds a certain percentage of a total wt %. Moreover, in the case of further mixing a binder for the purpose of increasing compatibility of the mixed material to improve the sound velocity, an appropriate kind and a weight ratio of the binder vary depending upon a kind and mixed ratios of the material into which the binder is mixed. Therefore, mixing the binder may just lower the sound velocity, and it has been difficult to select an appropriate binder in accordance with a material into which the binder is mixed.
As for document information on these prior arts, for example, Patent Document 1 is known.